In general, photosensitive printing plates are classified as planographic plates, intaglio plates and relief plates. The photosensitive planographic printing plate is produced by rendering the surface of a support hydrophilic by treating the surface either chemically or physically or by coating a hydrophilic polymer on the surface, followed by applying a suitable photosensitive material on the thus prepared hydrophilic surface.
The usual surface treatments include mechanical surface treating, such as brush graining, and chemical surface treating such as electrolytic graining and/or etching, and/or chemical surface treating which applies a further layer such as an alkali metal salt of phosphonic acid, a silicate, and potassium fluorozirconate, with or without anodic oxidation.
Most lithographic plates were once prepared from grained zinc plates which had been coated with a suitable photosensitive composition, dried, promptly exposed to secure the desired image, followed by applying a developing ink to the entire surface of the plate which was then washed with water to eliminate any water-soluble materials and developing ink. A gum arabic solution was thereafter applied to the printing surface of the plate to protect it until it was ready for use. The gum arabic provided chemical protection to the image and was easily washed off with water when it was desired to use the plate.
A planographic printing plate is described in U.S. Pat. No. 2,714,066 formed from a thin metal sheet having at least one surface thereof treated to provide a tightly bonded, thin, preferably inorganic, hydrophilic surface treatment, formed from a solution of an alkali metal silicate, salicyclic acid or other treating agent which would form a permanent hydrophilic scum-preventing and tone-reducing film overlying and in firmly bonded contact with the surface of the plate, and having a coating of a light-sensitive organic material over the thus treated surface. The preferred substrate is an aluminum foil or sheet material which has been cleaned, for example, by immersion in a solution of trisodium phosphate.
U.S. Pat. Nos. 3,511,661 to Rauner, as well as 3,860,426 and 3,920,457 to Cunningham et al., disclose coating anodized aluminum with carboxymethyl cellulose, but not in conjunction with diazo photosensitive layers, and utilizes procedures and additives not required in the present invention. However, Thomas in U.S. Pat. No. 3,549,365 utilizes an interlayer coating comprising derivatives of aromatic sulfonic acids.
The present invention is particularly concerned with presensitized plate systems in which the metal substrate has been prepared for application of the photosensitive material by anodization. A problem with known anodized presensitive plate systems has been the uncleanliness of the non-image areas during printing operations. This is a particularly serious problems with water developable plate systems. The natural porosity of the freshly anodized layers results in the absorption of materials of the photosensitive layer into the oxidized layer if the resulting layers are not sealed rendering the area hydrophobic causing ink and other impurities to adhere to the non-image areas. However, it is well known that the organic nature of the sensitizers, resins, additives and dyes may give rise to a shorter press life when such interlayers are employed.
One attempt to solve the problem addressed here was to treat the anodized presensitized plate with an aqueous solution of polyvinylphosphonic acid. This system retains the high printout and high contrast characteristics of the plate, generally eliminates the staining and generally improves the image deletion, water/ink balance (press tinting), exposure and shelf life. The press life, however, is about 25% reduced mainly due to sealing or interlayering chemicals with poor adhesion between the anodic oxide and the coating in the image area.
As previously set forth, U.S. Pat. No. 3,549,395 discloses the obtention of certain improvements when aromatic sulfonic acids are utilized instead of inorganic sealing or barrier-forming materials. Nevertheless, the patent prefers the use of sublayers and overlayers, and especially prefers both, when utilizing the aromatic sulfonic acids.
It is accordingly the object of this invention to provide a new planographic printing plate in which the non-image dirt problem is significantly overcome without substantially adversely affecting the press life and other desired characteristics of the printing plate.
This and other objects of the invention will become apparent to those skilled in the art from the ensuing description.